1. Field of the Invention
The invention relates to a device for transmitting optical signals between units that are rotatable relative to each other. Devices of this kind are used preferably in computer tomographs.
2. Description of the Prior Art
Various devices are known for transmitting optical signals between units that are rotatable relative to each other, particularly those having an unobstructed inner diameter. A basic problem existing here is that of designing a means for carrying light along the circumference of the device, and also suitable means for coupling light in and out. For use in computer tomographs, devices of this kind must have large unobstructed diameters of an order of magnitude of 1 meter. The circumferential speed of a rotation may be of an order of magnitude of 20 m/s. At the same time, data rates of more than 1 gigabit per second (Gbaud) must be feasible.
Thus, U.S. Pat. No. 4,109,997 discloses an optical rotating data transmission device in which travel of light along the circumference occurs by reflection at two opposite faces. Light guides or glass fibers are provided for coupling light in or out, a bundling or focusing of the light beam being effected by means of lenses. However, this device is attended by a whole series of disadvantages. Thus the optical attenuation of transmission is relatively high, owing to multiple reflections at relatively steep angles. Therefore, an optical transmitter is required to have high transmission power. Furthermore, the costs of fabrication are relatively high, owing to the confronting mirror-coated surfaces. Wideband data transmission with modulation-signal period-lengths which are substantially smaller than the transit time of the light around the circumference of the device is not possible, because a multiple-path reception of signals occurs at receiver positions close to a transmitter. Thus, signals received from the transmitter along a short path, and signals which have been reflected at least once around the circumference of the device, are received simultaneously. The transit time difference must be small in comparison with the period length of the modulation signal. Thus, with an inner diameter of about 1 meter, a total transit time around the circumference of about 10 nanoseconds results. For example, in a transmission of digital signals this makes it possible to achieve bit periods of maximally 50 nanoseconds, corresponding to a maximum transmission rate of 20 Mbaud.
An improvement of the optical system is disclosed in U.S. Pat. No. 4,525,025. This illustrates, particularly in FIG. 10, a specially suitable trench for transmitting optical signals. It consists only of one component part, and can therefore be manufactured at favorable cost. However, also this patent specification describes no effective solution of the problem of bandwidth limitation. In addition, the proposed coupling-in or coupling-out of light by blunt fiber ends can be achieved only with an extremely poor efficiency. Therefore, this device is suitable only for small diameters.
An improvement of optical coupling-in or coupling-out is disclosed in U.S. Pat. No. 4,555,631. In this, the coupling-in of optical signals into a mirror-finished cylinder is effected by means of two mirrors. For coupling-out, an additional coupling-out element is provided to be disposed at a fixed position in the trench. However, here too a large attenuation of the optical transmission path results, because the coupling-in mirrors cannot be placed arbitrarily close to the mirror-finished cylinder, in particular at high speeds of movement. Moreover, an unavoidable fanning of the light beam on the plane mirror surface results in an additional deterioration of efficiency. Furthermore, the mechanical construction of the coupling-out element involves much outlay, and is therefore liable to failure, and costly. Finally, the problem of bandwidth limitation is not solved. Thus, the light is conducted from a coupling-in position to a coupling-out position along two paths in opposite directions, and finally jointly evaluated in a receiver. Here too, the limitation applies that the period length of the modulation signal must be substantially smaller than the transit time of the light around the circumference of the device.
A device having an especially high optical efficiency is described in U.S. Pat. No. 4,934,783. In this, a focusing of the beam of rays is effected through a lens system. However, this system involves much outlay, is expensive to fabricate, and is suitable only for small diameters. Furthermore, here too the wideband problem has not been solved.
In order to reduce the attenuation along the transmission path, and to increase the bandwidth capable of being transmitted, U.S. Pat. No. 6,104,849 proposes a transmission along a plurality of shortened segments. A reduced attenuation results from the shortened segments. Here the maximum bandwidth is inversely proportional to the length of the segments. Therefore a larger bandwidth can be achieved with shorter segments. However, for this a correspondingly larger number of optical transmitters or receivers is needed to cover the entire circular circumference. Thus, the system costs rise in proportion to the band width.
DE 195 43 386 C1 describes a wideband signal transmission device which makes possible a large bandwidth, but gives no indication of transmission with high transmission quality.
U.S. Pat. No. 4,962,986 describes an alternative device for coupling light. For coupling light into and out of light-guiding fibers, a coupling medium having a higher refractive index than the surroundings is directly contacted with a fiber core. With this, a deflection of the light carried in the fiber into the coupling medium is effected. This arrangement has the decisive disadvantage that the coupling medium must directly contact the fiber core. Therefore this system is adapted for use almost exclusively for coupling at given fixed locations. A system of this kind is, however, hardly applicable to arrangements in which a transmitter and a receiver move relative to each other, because here the coupling medium must move at a high speed along a usually very thin and sensitive fiber core.
A device of this kind is described by Tamir, “Integrated Optics,” published by Springer Verlag, Berlin, 1979, page 87. In this, a prism serving for coupling-out is positioned at as small as possible a distance above the fiber core. In order to obtain a reasonable coupling efficiency here, the distance between the prism and the fiber core must be of the order of magnitude of a light wavelength. However, with conventional highly precise bearing means, this accuracy can be achieved only with small dimensions of the entire arrangement. Therefore at present this system cannot be used, for example in computer tomographs having a diameter of 1.5 meters and circumferential speeds of up to 20 m/s.